Electrical resistance heating elements are used in many industrial, commercial and residential applications. They have been used to heat electroplating baths in the plating industry and can be found in the baseboard heaters of many homes. A typical construction for an electrical resistance heating element includes a pair of terminal pins brazed to the ends of a Ni--Cr coil, which is then axially disposed through a U-shaped tubular metal sheath. The resistance coil is insulated from the metal sheath by a powdered ceramic material, usually magnesium oxide.
More modern heating elements have been-developed using polymeric insulating materials around a resistance heating wire, such as disclosed in U.S. Pat. No. 5,586,214. These more recent devices employ resinous coatings which are often injection molded over the resistance wire. Since resistance wire is often extremely pliable, injection molding pressures are known to distort the circuit pattern for the wire in unacceptable ways. One solution described in the '214 patent is to provide a polymer inner mold having a series of grooves for receiving the wire and holding it in place while a thermoplastic coating is injection molded over the assembly. This technique has been difficult to implement when thermoplastic materials are loaded with significant amounts of ceramic additives. Such mixtures are viscous and require great pressures of 10,000-25,000 psi to fill the mold properly. Even high mold pressures are sometimes insufficient to fill the details of the mold properly and the greater the mold pressure, the more stress is applied to the circuit pattern.
In still a further method described in U.S. Pat. No. 3,968,348, a resistance heating wire is disposed between a pair of fiberglass mats. A third fiberglass mat carries a heat dissipating metal foil. The three mats are separately impregnated with a thermosetting polyester resin and cured together to form a rigid, fluid impervious laminated structure. While laminating techniques have occasionally produced acceptable products, they often leave air gaps in the cross-section which make uniform heating difficult. Additionally, insufficient bonding to the glass mats or resistance wire can cause delamination, especially due to the difference in thermal expansion rates during heating and cooling cycles.
While such methods for creating resistance heating elements with thermoplastic or thermosetting polymers are known, there remains a need for better manufacturing processes which can further reduce the cost and improve the quality of polymer heaters. There also remains a need for more structural integrity during heating cycles and more effective thermal dissipation of heat from the resistance wire.